Method of fabricating shape memory films

ABSTRACT

A method for fabricating a shape memory polymer into a three-dimensional object is provided. The method includes forming a film of crosslinked poly(amic acid) on a substrate to provide a laminated substrate; forming the laminated substrate into a first configuration that is in a three-dimensional form; curing the cross-linked poly(amic acid) to provide the shape memory polymer having a permanent shape corresponding to the first configuration; and removing the substrate from the laminated substrate to provide the three-dimensional object comprising the shape memory polymer. The formation of the laminated substrate into the three-dimensional object may be based on origami techniques.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 62/202,293, which was filed on Aug. 7, 2015 and isincorporated herein by reference in its entirety. This application isalso related to co-pending U.S. patent application Ser. No. 15/009,360,filed on even date herewith, by inventor Loon-Seng Tan, et al., andentitled “MULTIFUNCTIONAL CROSSLINKING AGENT, CROSSLINKED POLYMER, ANDMETHOD OF MAKING SAME,” which is incorporated herein by reference in itsentirety.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

FIELD OF THE INVENTION

The invention generally relates to methods of making films that possessshape memory properties at elevated temperatures, and more particularlyto methods of fabricating crosslinked polyimide or poly(amide-imide)polymer films that have shape memory properties.

BACKGROUND OF THE INVENTION

Shape memory polymers (SMPs) are a class of active materials that can beprogrammed to “fix” a temporary shape or a series of temporary shapes,and then later to recover to a “memorized” permanent shape establishedby a covalent or physical network by applying thermal, electrical, orother environmental stimulus. In the case of thermal stimulation, theshape memory polymers are characterized by deforming at a temperatureabove a softening transition such as glass transition temperature(T_(g)) and melting point (T_(m)) of the polymer, then cooling throughthis transition under stress, causing immobilization of the constituentnetwork chains, and the macroscopic shape to be fixed. Recovery of thepermanent shape is then accomplished by heating above the triggeringtemperature, which re-mobilizes the network chains and allows rubber(entropic) elasticity to return the sample to its equilibrium shape.Depending on the nature of external stimulus, SMPs are categorized asthe thermally-induced SMPs, light-induced SMPs, electro-active SMPs,pH-responsive SMPs, and water/moisture-driven SMPs, and variousmechanisms are operating in each category. Shape-memory materials, whichinclude shape-memory alloys (SMAs), have been widely used in actuation,aircraft, robotics, piping, medical and dental applications. It is notedthat SMPs differ from SMAs in that their glass transition or meltingtransition from a hard to a soft phase, which is responsible for drivingthe shape memory effect, whereas for SMAs, martensitic/austenitictransitions are responsible for the shape memory effect.

There are numerous advantages that make SMPs more attractive than SMAs.For example, SMPs have much higher capacities for elastic deformation(up to 200% in most cases), much lower cost, lower density, a broaderrange of application temperatures which can be tailored, comparativelyeasy processing, and potential biocompatibility and biodegradability.However, the state-of-the art SMPs are consisted of high-alkyl contentpolymers such as, polyurethane, poly(ε-caprolactone), poly(norbornene),(ethylene-oxide)/(ethylene terephthalate)-based copolymers,styrene/butadiene copolymers, thiol-ene/acrylate copolymers, etc.Therefore, none of the foregoing has shape-memory propertiestemperatures above 150° C. or long-term thermal and thermo-oxidativestabilities in this temperature region. Accordingly, for extremely hotenvironment applications, current state-of-the art shape-memorymaterials lack key properties that enable high-temperaturepatterning/processing, and sustaining performance, dimensionalstability.

In recent years, a number of high-temperature shape-memory polymers haveappeared in open and patent literature. For example, aromaticpolyimides, polyamides, and poly(amide-imide)s are common classes ofheat-resistant, thermally stable, polymers with glass transitiontemperatures in excess of 150° C. Recent work has revealed thatcrosslinking these polymers with certain crosslinking agents (e.g., tri-and tetra-amines and tri- and tetra-anhydrides) can impart shape-memoryeffects. For example, multi-functional amine crosslinking agents havebeen described in U.S. Pat. Nos. 8,546,614; 8,791,227; and 8,962,890,and multi-functional anhydride crosslinking agents are described in U.S.Provisional Patent Application entitled MULTIFUNCTIONAL CROSSLINKINGAGENT, CROSSLINKED POLYMER, AND METHOD OF MAKING SAME, filed on evendate herewith. Each of the foregoing U.S. patent documents isincorporated herein by reference in its entirety.

Despite the foregoing, there are few described methods for fabricatingthese heat-resistant, thermally stable SMPs into useable objects.Accordingly, there is a need for new methods of fabricating objects fromSMPs.

SUMMARY OF THE INVENTION

In accordance with embodiments of the present invention, a method forfabricating a shape memory polymer into a three-dimensional object isprovided. The method includes a) forming a solution comprising apoly(amic acid) intermediate, wherein the solution comprises a solvent,and a reaction product of an aromatic diamine monomer and adi-anhydride-containing monomer; b) treating the solution comprising thepoly(amic acid) intermediate with a multi-functional crosslinking agentto thereby form a sol-gel comprising a crosslinked poly(amic acid); c)forming a film of crosslinked poly(amic acid) on a substrate to providea laminated substrate; d) forming the laminated substrate into a firstconfiguration that is in a three-dimensional form; e) curing thecross-linked poly(amic acid) to provide the shape memory polymer havinga permanent shape corresponding to the first configuration, wherein theshape memory polymer comprises a crosslinked polyimide or a crosslinkedpoly(amide-imide); and f) removing the substrate from the laminatedsubstrate to provide the three-dimensional object comprising the shapememory polymer.

In accordance with another embodiment, the method further includes g)heating the three-dimensional object to a first temperature that isabove a triggering temperature; h) deforming the three-dimensionalobject to a second configuration that is different from the firstconfiguration; and i) lowering the three-dimensional object to a secondtemperature that is below the triggering temperature while thethree-dimensional object is maintained in the second configuration.

In yet another embodiment, the method further includes j) heating thethree-dimensional object in the second configuration to a thirdtemperature that is above the triggering temperature to thereby inducethe self-rearrangement of the three dimensional object from the secondconfiguration to the first configuration.

In yet another embodiment, the multi-functional crosslinking agentutilized in the method is a multi-functional anhydride crosslinkingagent, and the poly(amic acid) intermediate is an amine-terminatedpoly(amic acid) intermediate obtained by reacting a stoichiometricexcess of the aromatic diamine monomer with the di-anhydride-containingmonomer. Complementary to the foregoing and in yet another embodiment,the multi-functional crosslinking agent utilized in the method is amulti-functional amine crosslinking agent, and the poly(amic acid)intermediate is an anhydride-terminated poly(amic acid) intermediateobtained by reacting a stoichiometric excess of thedi-anhydride-containing monomer with the aromatic diamine monomer.

Additional objects, advantages, and novel features of the invention willbe set forth in part in the description and drawings which follows, andin part will become apparent to those skilled in the art uponexamination of the following or may be learned by practice of theinvention. The objects and advantages of the invention may be realizedand attained by means of the instrumentalities and combinationsparticularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with the summary given above, and the detailed descriptiongiven below, serve to explain the invention.

FIG. 1 illustrates processing chemistry that is used to generate acrosslinked polyimide film from its corresponding reactive polyamic acidand either a multi-anhydride or multi-amine crosslinking agent, inaccordance with an embodiment of the present invention;

FIG. 2A illustrates a flow chart for the construction of a six-sided boxvia poly(amic acid)/crosslinking/polyimide processing chemistry, inaccordance with another embodiment of the invention;

FIG. 2B illustrates the high temperature shape memory effect of thecrosslinked polyimide film shaped into a cube, in accordance withanother embodiment of the invention;

FIG. 2C illustrates the high temperature shape memory effect of thecrosslinked polyimide film shaped into a square-based pyramid, inaccordance with another embodiment of the invention;

FIG. 2D illustrates the high temperature shape memory effect of thecrosslinked polyimide film shaped into a “paper airplane,” in accordancewith another embodiment of the invention;

FIG. 3 is a plot of Storage Modulus (MPa) versus Temperature (° C.) fora linear polyimide and various crosslinked polyimides demonstratingrelative dimensional stability of the glass-transition plateau;

FIG. 4 is a three dimensional plot of Strain (%) versus Static Force (N)versus Temperature (° C.) showing a data from a shape memorystress-strain-temperature demonstration, which was processed inaccordance with an embodiment of the present invention; and

FIGS. 5A and 5B are two-dimensional plots of Strain (%), Static Force(N), and Temperature (° C.) versus time showing data from 1 cycle to 21cycles (5A) and 1 cycle to 5 cycles (5B) from a demonstration of changeof strain, stress, with temperature and time using a sample film ofPAn-CP2-5, which was processed in accordance with an embodiment of thepresent invention

DETAILED DESCRIPTION OF THE INVENTION

In accordance with embodiments of the present invention, a method forfabricating a shape memory polymer into a three-dimensional object isprovided. The method includes a) forming a solution comprising apoly(amic acid) intermediate, wherein the solution comprises a solvent,and a reaction product of an aromatic diamine monomer and adi-anhydride-containing monomer; b) treating the solution comprising thepoly(amic acid) with a multi-functional crosslinking agent to therebyform a sol-gel comprising a crosslinked poly(amic acid); c) forming afilm of crosslinked poly(amic acid) on a substrate to provide alaminated substrate; d) forming the laminated substrate into a firstconfiguration that is in a three-dimensional form; e) curing thecross-linked poly(amic acid) to provide the shape memory polymer havinga permanent shape corresponding to the first configuration, wherein theshape memory polymer comprises a crosslinked polyimide or a crosslinkedpoly(amide-imide); and f) removing the substrate from the laminatedsubstrate to provide the three-dimensional object comprising the shapememory polymer.

The method may further include g) heating the three-dimensional objectto a first temperature that is above a triggering temperature; h)deforming the three-dimensional object to a second configuration that isdifferent from the first configuration; and i) lowering thethree-dimensional object to a second temperature that is below thetriggering temperature while the three-dimensional object is maintainedin the second configuration. Furthermore, the method may include j)heating the three-dimensional object in the second configuration to athird temperature that is above the triggering temperature to therebyinduce the self-rearrangement of the three dimensional object from thesecond configuration to the first configuration.

According to another embodiment of the present invention, the formationof the laminated substrate may be based on origami techniques. Ascommonly known, origami is a Japanese art based on folding paper, but inrecent years, the concept has been advanced beyond artistic creationsand toys such that three-dimensional, complex objects now can bedesigned to be capable of on-command transformation into a wide range ofdevices and robotic systems. Easily taken for granted examples toillustrate the utility of origami concept can be found in foldable maps,shopping bags, storage boxes and cartons, etc. More advanced examplessuch as automobile airbags, shock absorbers, 3D and light-trappingphotovoltaics, and biomedical devices/implants such as stent furtherillustrate the ingeneous application of origami engineering concept. Ingeneral, origami concept is best used to solve technological problemsthat require solutions to (i) small-volume packaging for (ii) efficientstorage and (iii) transportation, (iv) easy deployment, and in somecases (v) reusability. The most attractive features of origami conceptare (i) it is scale-free, applicable from nanoscale level (proteinfolding and DNA origami) to kilometer-scale (solar panels) and (ii)applicable to various printing techniques.

Fundamentally, origami can be considered as a process that involves asequence of folding steps (i.e. programmed fold or crease pattern) toeventually transform a 2-D substrate to the designed 3-D object.Therefore, important to 2D-to-3D transformation process is shape memoryeffect, and naturally, a requisite characteristic of the polymersubstrate for origami-inspired fabrication is to have a shape-memorycapability. The notion of imparting elastomer-like shape memory effectto thermoplastic or crosslinked polymers may be based on three differentmechanisms: thermal, photothermal, and photochemical mechanisms.

For extremely hot environment applications, current state-of-the art(SOA) shape-memory materials lack key properties that enablehigh-temperature patterning/processing, and sustaining performance, viz.dimensional stability. Accordingly, the disclosed fabrication method isbased on poly(amic-acid)/polyimide chemistry to constructorigami-inspired, deployable objects, which can rapidly transform fromflat structures to 3D shapes at temperatures in excess of 200° C.Additionally, the flat structures that embody temporary configurationhave been observed to be dimensionally stable under ambient conditions.

Crosslinked Polyimide and Poly(Amide-Imide) Polymers

Because of the similar polymerization chemistry to generate polyimidesand poly(amide-imides), the multi-functional crosslinking agentsdisclosed herein may be used to crosslink these classes of polymers tocreate covalent network structures capable of showing shape memoryeffects at elevated temperatures.

Synthesis of a polyimide is typically accomplished by polymerization ofa diamine and a dianhydride in a 1:1 molar ratio to generate a poly(amicacid) precursor, which is then converted to the corresponding polyimidetypically by either thermal cure (e.g., by heating to >200° C. insolution or solid state) or chemical imidization using a dehydratingagent or promoter such as acetic anhydride/triethylamine or aceticanhydride/pyridine. However, to generate a polyimide having the desiredamount of crosslinking, an appropriately-terminated poly(amic acid)precursor is first generated by off-setting the dianhydride:diamineratio. For example, to provide an amine-terminated poly(amic acid)precursor, the amount of diamine is used in excess to cap both ends ofthe poly(amic acid) precursor. An appropriate amount of amulti-anhydride crosslinking agent is then added to the precursorsolution so that all or substantially all of the terminal amine groupswill be consumed. Conversely, to provide an anhydride-terminatedpoly(amic acid) precursor, the amount of di-anhydride-containing monomeris used in excess to cap both ends of the poly(amic acid) precursor.Then an appropriate amount of a multi-amine crosslinking agent is thenadded to the precursor solution so that all or substantially all of theterminal anhydride groups will be consumed. In either embodiment,crosslinked polyimides may then be created using appropriate imidizationconditions.

In accordance with an aspect of the polymer, the diamine monomercomprises an aromatic diamine, which includes, but is not limited to,1,3-bis(3-aminophenoxy)benzene (APB); 1,4-bis(3-aminophenoxy)benzene;1,2-bis(3-aminophenoxy)benzene; 1,2-bis(4-aminophenoxy)benzene;1,3-bis(4-aminophenoxy)benzene; 1,4-bis(4-aminophenoxy)benzene;3,4′-oxydianiline; 4,4-oxydianiline; 1,3-diamino-4-methylbenzene;1,3-diamino-4-(trifluoromethyl)benzene; 2,4-diaminobiphenyl;2,2-bis(4-aminophenyl)-1,1,1,3,3,3-hexafluoropropane;2,2-bis(4-aminophenyl)propane;2,2-bis[4-(4-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane;2,2-bis[4-(4-aminophenoxy)phenyl]propane; or a mixture of thereof.

In accordance with another aspect, the dianhydride monomer includes, butis not limited to 2,2-[bis(4-phthalicanhydrido)]-1,1,1,3,3,3-hexafluoroisopropane (6FDA);4,4′-oxybis(phthalic anhydride); 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride; 3,3′,4,4′-benzophenone tetracarboxylicdianhydride; 4,4′-(2,2,2-trifluoro-1-phenylethylidene)bis[phthalicanhydride]; 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride;4,4′-(p-phenylenedioxy)bis[phthalic anhydride];4,4′-(m-phenylenedioxy)bis[phthalic anhydride];4,4′-(o-phenylenedioxy)bis[phthalic anhydride]; or mixtures thereof.

The synthesis of a poly(amide-imide) is typically accomplished bypolymerization of i) a diamine and a trimellitic anhydride (TMA) ortrimellitic anhydride acid chloride (TMAC); or ii) a diamine and adiimide-dicarboxylic acid monomer derived from a selective condensationof TMA and an aromatic diamine (e.g., H₂N—Ar—NH₂). When acid monomersare used, the polymerization process is aided bytriethylphosphite/pyridine (Yamazaki-Higashi reagent) in a 1:1 molarratio in an amide solvent such as N,N-dimethylformamide (DMF),N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), etc.

Persons having ordinary skill in the art will appreciate that thesepolymerization methods may be applied to other dianhydride monomerscontaining preformed aromatic amide moieties. For example, bis(phthalicanhydride) monomers with preformed amide as part of the linking group,which are also known as diamide-dianhydrides (DADA), can be preparedfrom trimellitic anhydride acid chloride (TMAC) and an aromatic diamine(e.g., H₂N—Ar—NH₂) in refluxing acetone with pyridine as HCl scavenger.

However, to generate a poly(amide-imide) having the desired amount ofcrosslinking, an appropriately-terminated poly(amide-imide) may be firstgenerated via Yamazaki-Higashi reaction by off-setting TMA:diamineratio. To make the amine-terminated poly(amide-imide), the amount ofdiamine is in excess to cap both ends of the polymer. After theamino-terminated polyamide has been isolated by precipitation inmethanol and filtration, it is washed with methanol and dried in vacuoovernight. The amino-terminated polyamide can then be dissolved orsuspended in an amide solvent and mixed with an amide solution of amulti-anhydride crosslinking agent in an appropriate amount so that allor substantially all of the terminal amine groups are consumed, which isimmediately followed by casting and thermal curing under reducedpressure to create crosslinked poly(amide-imide) films. Conversely, tomake the anhydride-terminated poly(amide-imide), the amount of aminemonomer is not used in excess and thus is the limiting reagent.

In a preferred method to generate a poly(amide-imide) having the desiredamount of crosslinking, it is more suitable that anappropriately-terminated poly(amide-amic acid) is first generated via byoff-setting DADA:diamine ratio so that either diamide-dianhydride ordiamine is in excess. In the former case, anhydride-terminatedpoly(amide-amic acid) is generated, and in the latter case,amine-terminated poly(amide-amic acid). Then an appropriate amount of amulti-amine crosslinking agent or multi-anhydride crosslinking agent isthen added to the respective precursor solution so that all orsubstantially all of the reactive terminal groups will be consumed. Thisis immediately followed by casting and thermal curing under reducedpressure to create crosslinked poly(amide-imide) films.

The ability to vary the amount of crosslinking allows the synthesis ofcrosslinked polyimides and poly(amide-imides) with mechanical properties(i.e. T_(g)) tailored to a specific application or to specificenvironmental conditions. A generalized method for incorporating asufficient amount of functionalization in the poly(amic acid)-containingprecursor may be based on the desired degree of crosslinking in theresultant polyimide or poly(amic acid). For example, the general methodcan include selecting the desired multi-functional crosslinking agent(e.g., amine- or anhydride-functionalized) and the desired amount ofcrosslinking (x mol %), and then preparing the appropriatelyfunctionalized poly(amic acid)-containing precursor by employing theexcess monomer in an amount of about (100+1.5 x) mol %. The limitingreagent is based on 100 mol %. In one embodiment, the amount ofmulti-functional crosslinking agent used to crosslink the poly(amicacid) may vary from about 0.3 mol % to about 10 mol %. In anotherembodiment, the amount of multi-functional crosslinking agent used tocrosslink the poly(amic acid) may vary from about 0.5 mol % to about 5mol %. For example, the multi-functional crosslinking agentconcentration about 0.5 mol %, about 1.0 mol %, about 2.0 mol %, orabout 5.0 mol %.

Multi-Anhydride Crosslinking Agents

In accordance with an embodiment of the present invention, amultifunctional crosslinking agent is provided that has three or fouranhydride functional groups (i.e., tri-anhydride crosslinking agents ortetra-anhydride crosslinking agents, respectively). The multifunctionalcrosslinking agents may be defined by the general chemical formula (I):Z—(—Ar—)_(n)—W, wherein Z represents an anhydride functional group thatis directly or indirectly bonded to Ar; wherein Ar represents an arylgroup that is directly bonded to W; and wherein n is equal to 3 when Wis N or P═O, or n is equal to 4 when W is Si or an sp³ hybridized carbonmoiety. Accordingly, the tri-anhydride crosslinking agents may beZ—(—Ar—)₃N or Z—(—Ar—)₃P═O; and the tetra-anhydride crosslinking agentsmay be Z—(—Ar—)₄Si or Z—(—Ar—)₄R, where R represents the carbon moiety(e.g., fluorenyl).

In accordance with an aspect of the invention, the aryl group (Ar),which is directly bonded to W through a carbon atom, may be aphenyleneoxy group (—OPh-) that is para- or meta-substituted withrespect to oxygen, and thus the agent may be further defined by thegeneral chemical formula (II): Z—(—OPh-)_(n)—W. In one embodiment, thephenyleneoxy group represents that the benzene ring portion mayunsubstituted (i.e., OC₆H₄); alternatively, the phenyleneoxy group mayhave substituents (e.g., C1-C4 alkyl groups) around the benzene ring. Inone example, where W is P═O, the agent may be further defined by thegeneral chemical formula (III): Z—(—OPh-)₃—P═O.

In accordance with another aspect, the aryl group (Ar) may be aphenyleneoxy group (—OPh-) that is para- or meta-substituted withrespect to oxygen, and Z may be a phthalic anhydride group that isconnected to the phenyleneoxy group through an ether bond. Accordingly,the agent may be further defined by the general chemical formula (IV):

wherein W and n are as defined above, and R¹ through R⁸ areindependently selected from H or C1-C4 alkyl. Where W is P═O, thesephosphine oxide-based crosslinking agents may be defined by the generalchemical formula (V):

Exemplary phosphine oxide-based crosslinking agents aretris[4-(3,4-dicarboxyphenoxy)phenyl] phosphine oxide tri-anhydride(where R¹ to R³ and R⁵ to R⁸ are H), and its meta-isomer, which istris[3-(3,4-dicarboxyphenoxy)phenyl] phosphine oxide tri-anhydride(i.e., where R¹ to R⁴ and R⁶ to R⁸ are H).

In accordance with yet another aspect, where W is N, and where the arylgroup (Ar) is the phenyleneoxy group (—OPh-), which is para- ormeta-substituted with respect to oxygen, a tertiary amine-basedcrosslinking agent may be defined by the general chemical formula (VI):Z—(—OPh-)₃N. In another embodiment, where Z is the phthalic anhydridegroup that is connected to the phenyleneoxy group through an ether bond,exemplary tertiary amine-based crosslinking agents may be defined by thegeneral chemical formula (VII):

Exemplary tertiary amine-based crosslinking agents aretris[4-(3,4-dicarboxyphenoxy)phenyl]amine tri-anhydride (where R¹ to R³and R⁵ to R⁸ are H), and its meta-isomer, which istris[3-(3,4-dicarboxyphenoxy)phenyl]amine tri-anhydride (i.e., R¹ to R⁴and R⁶ to R⁸ are H).

In accordance with yet another aspect where W is nitrogen (N), and whereZ and Ar from Formula (I) in combination form a phthalic anhydridemoiety that is directly bonded to nitrogen, this tertiary amine-basedcrosslinking agent may be defined by the general formula (VIII):

wherein R⁹ to R¹¹ are independently selected from H or C1-C4 alkyl. Anexemplary tertiary amine-based crosslinking agent is tris(phthalicanhydride)amine (where R⁹ through R¹¹).

In accordance with another embodiment, complementary tetra-anhydridecrosslinking agents include where W (in Formulas (I), (II), and (IV)) isa carbon moiety, and thus n is 4. Further, exemplary carbon-basedtetra-anhydride crosslinking agents 4a, 4b, and 5c are also shown inTable 1. The carbon moiety may include a centralized sp³ hybridizedcarbon to provide a generally tetrahedral geometry to the agent. Forexample, agents 4a and 4b include a fluorenyl group, where the C9 carbonof the fluorenyl group is sp³ hybridized.

As also shown in Table 1, the complementary tetra-anhydride crosslinkingagents include where W (in Formulas (I), (II), and (IV)) is silicon oran sp³ hybridized carbon, and thus n is 4. Further, exemplarysilicon-based (5a and 5b, E=Si) or sp³ hybridized carbon-based (5c and5d, E=C) tetra-anhydride crosslinking agents are also shown in Table 1.

TABLE 1 Exemplary Multi-Anhydride Crosslinking Agents Tri-anhydridecrosslinkers Tetra-anhydride crosslinkers

In accordance with an embodiment, the cross-linked polyimides obtainedwith the multi-anhydride crosslinking agents may be defined by thefollowing general chemical formula (IX):

where W may be P═O, N, Si, or a carbon moiety (e.g., fluorenyl); whereinL denotes either a direct covalent bond to W or a linking group (e.g., aphenyleneoxy group) for indirect bonding to W; n, m, l denote the degreeof polymerization (DP) of each branch of polyimide, which may be of thesame or different values, with the DP range of about 3 to about 30. Forexample, in an embodiment, DP is in a range of about 5 to about 25, orabout 10 to about 20. The overall network structure is denoted by theinfinity symbol (∞). The linking group Y is one of the followingmoieties: —C(CF₃)₂—, —O—, —SO₂—, —C(═O)—, -(Ph)C(CF₃)—,—OPh-C(CF₃)₂—OPh-, —OPh-C(CH₃)₂—OPh-. In another embodiment, Y is—C(CF₃)₂.

In accordance with another embodiment, the cross-linked polyimidesobtained with the multi-anhydride crosslinking agents may be defined bythe following general chemical formula (X):

where W may be N or P═O; L, n, m, l, and Y are as defined above.

In accordance with yet another embodiment, the cross-linked polyimidesobtained with the multi-anhydride crosslinking agents may be defined bythe following general chemical formula (XI):

where W is N or benzene-1,3,5-trioxy (1,3,5-C₆H₃O₃); and n, m, l, and Yare as defined above.

Multi-Amine Crosslinking Agents:

In accordance with an embodiment of the present invention, amultifunctional crosslinking agent is provided that has three aminefunctional groups (i.e., tri-amine crosslinking agent). The tri-aminecrosslinking agent may be defined by the general chemical formula (IX):(H₂N—Ar—)₃—W, wherein Ar represents an aryl group that is directly orindirectly bonded to W; and wherein W may be CH₃C (methylcarbyl); N(trivalent nitrogen); P═O (phosphine oxide); or BO₃ (borate).Accordingly, the tri-amine crosslinking agents may be (H₂N—Ar—)₃—CCH₃,(H₂N—Ar—)₃—N, (H₂N—Ar—)₃—P═O, or (H₂N—Ar—)₃—BO₃. In an embodiment, theAr is a biaryl ether, and thus the tri-amine crosslinking agent may befurther defined by the general formula (X): (H₂N—Ar′—O—Ar″—)₃—W, whereAr′ and Ar″ may be similarly or differently substituted, and where thevarious isomers are further contemplated.

According to yet another embodiment, the tri-amine crosslinking agent isa tri(oxybenzene-amine) crosslinker having the following general formula(XII):

wherein W may be CH₃C (methylcarbyl); N (trivalent nitrogen); P═O(phosphine oxide); or BO₃ (borate); R may be H, F, Cl, CF₃, or CH₃; andthe amine groups (—NH₂) may be in the meta or para position with respectto oxygen of the biaryl ether bond. Exemplary tri(oxybenzene-amine)crosslinking agents 6a,b; 7a,b; 8a,b; and 9a,b are shown in Table 2.

TABLE 2 Exemplary Tri(oxybenene-amine) Crosslinking Agents

Exemplary crosslinked aromatic polyimides obtained from thetri(oxybenzene-amine) crosslinking agents (where R═H) have the followinggeneral formula (XIII):

wherein Y is selected from the group consisting of —C(CF₃)₂—, —O—,—SO₂—, —C═O—, -(Ph)C(CF₃)—, —OPh-C(CH₃)₂-PhO—, —O(1,3-Ph)O— and—O(1,4-Ph)O—; n, m, and l are degrees of polymerization (DP) of eachbranch of the crosslinked aromatic polyimide; and the infinity symbol(∞) is used to denote an infinite network structure for a crosslinkedpolymer.

Similar to the crosslinked polymers obtained using the multi-anhydridecrosslinking agents, the degrees of polymerization (DP) of each branchof the crosslinked aromatic polyimide may be the same or different. Inone exemplary embodiment, the DPs are the same with respect to oneanother. In another embodiment, at least one of the DPs is different. Inanother embodiment, the DP of each branch may be in a range of about 3to about 110 units. In an alternative embodiment, the DP may be in arange of about 3 to about 30, or about 5 to about 55 units. For example,in another embodiment, DP is in a range of about 5 to about 25, or about10 to about 20.

The extent and amount of crosslinking in the crosslinked polyimidepolymers and films may be altered by varying the concentration of thetri-amine crosslinker (i.e. about 0.5 mol %, about 1.0 mol %, about 2.0mol %, or about 5.0 mol %). In one embodiment, the tri-amine crosslinkerconcentration may vary from about 0.3 mol % to about 10 mol %. Inanother embodiment, the tri-amine crosslinker concentration may bebetween about 0.5 mol % to about 5 mol %. The ability to vary the amountof crosslinking allows the synthesis of crosslinked polyimides withmechanical properties (i.e. T_(g)) tailored to a specific application orto specific environmental conditions.

EXAMPLES

The following examples and methods are presented as illustrative of thepresent invention or methods of carrying out the invention, and are notrestrictive or limiting of the scope of the invention in any manner.

With reference to FIG. 1, exemplary processing chemistry that is used togenerate either tri-anhydride or tri-amine crosslinked polyimide filmsfrom their respective reactive polyamic acid solution and crosslinkerare provided. CP2 (LaRC™—CP2, NASA Langley Research Center) is anexemplary fluorinated polyimide derived from 2,2-bis(4-phthalicanhydrido)-1,1,1,3,3,3-hexafluoroisopropane (6FDA) and1,3-bis(3-aminophenoxy)benzene (APB). The subject polyimide (CP2) isselected to prove the concept because it is a well-known andwell-characterized polyimide derived from 2,2-bis(4-phthalicanhydrido)-1,1,1,3,3,3-hexafluoroisopropane (6-FDA, a dianhydridemonomer) and 1,3-bis(3-aminophenoxy)benzene (APB, a diamine monomer).Briefly, CP2 is a high-performance aerospace-grade polyimide thatpossesses remarkable properties including, high mechanical toughness,solvent resistance, high glass transition temperature, ultravioletradiation resistance, low color, low solar absorption, and high thermaland thermo-oxidative stability. CP2 is particularly suitable forlong-term survivability in space environments, and has been used todevelop lightweight, inflatable structures that serve as Gossamer-likespacecraft, satellites, and solar energy collection/reflection systems.Addition of high-temperature shape-memory capability to CP2 and relatedpolyimides will extend their applications where robust, dynamicproperties are required under extremely hot conditions.

Still referring to FIG. 1, the origami-inspired fabrication process isbased on the processing chemistry of poly(amic acid)/polyimide usingeither a triphenylphosphine-based trianhydride (4a) or triamine (4b) asa crosslinker. In this process, when the trianhydride crosslinker (x mol%) is used (i.e. process A), the co-monomers, APB (a diamine, 1) and6FDA (a dianhydride, 2) were dissolved under nitrogen atmosphere in apolar aprotic solvent such as N,N-dimethylacetamide (DMAc) (5 wt %polymer concentration) at room temperature for 24 h with excess APB(i.e. 1.5x mol % excess where x=mol % of trianhydride crosslinker used)for the preparation of poly(amic acid) oligomers (PAA oligomers, 3a)with reactive amine function as endgroups. Subsequently, thetrianhydride crosslinker (e.g. phosphine oxide trianhydride, 4a; x mol%) was added to the solution of PAA oligomers with a reactionstoichiometric ratio of the amino group to the terminal acid anhydride.After the crosslinker had completely dissolved, the resulting PAAsol-gel (5a) was immediately used in the fabrication of origami objectas described in the following paragraph. Alternatively in the process B,when a triamine crosslinker is used, anhydride-terminated PAA (3b)solution is generated from the initial polymerization mixture of excessdianhydride monomers and diamine monomer, followed by addition ofstoichiometrically balanced amount of the triamine crosslinker to resultin the modified PAA sol-gel (5b) for immediate used in the fabricationof an origami object.

Referring to FIG. 2A, a flow chart for constructing a hollow shapememory polyimide cube is provided. The first step of the fabrication isto prepare the modified PAA solution (sol-gel) containing the requisiteamount of an appropriate crosslinker as described above and shown inFIG. 1. The second step entails pouring the viscous PAA sol-gel into asubstrate (e.g. an aluminum dish). The third step pertains toevaporation of the solvent under reduced pressure and in temperaturerange where none or partial curing of PAA is taking place. For example,the substrate coated with the viscous modified PAA solution may beheated to a temperature from about 50° C. to about 100° C. under reducedpressure, such that only partial curing of PAA is taking place. Howeveruse of lower temperatures (e.g., room temperature to about 50° C.) andlower pressures (e.g., less than about 200 torr or less than about 100torr) may minimize or prevent excessive curing. Once the laminatedsubstrate is stable, the next step involves drawing the origami foldingpattern on the aluminum side of the substrate and cut out the patternwith a pair of scissors. Manual folding sequence of the two-dimensionalconfiguration of the origami object at room temperature is thenperformed so that the aluminum substrate is on the outside to form athree-dimensional form.

Following an imidization process, where a curing schedule forcrosslinked polyimides is performed (e.g., sequential heating at about150° C., about 175° C., about 200° C., about 250° C., and/or about 300°C.), the next step involves removal of the substrate. For examplealuminum can be dissolved easily in aqueous HCl. The resultingcrosslinked polyimide hollow cube is shown in the top photo of FIG. 2B.

Still referring to FIG. 2B, this crosslinked polyimide cube can beunfolded manually into the corresponding planar “cross” structure in anoil bath at about 215° C. Stable indefinitely at room temperature, thetemporary “cross” structure spontaneously (approximately less than about20 seconds) folds into the cube upon immersion in (by dropping into) thesame hot oil bath. The PI cube showed no visible shape distortion after3 days at 215° C., and regained its initial modulus (˜3 GPa by dynamicmechanical analysis) after taken out of the oil bath. As depicted inFIGS. 2C and 2D, two more origami objects have been similarlyfabricated, namely a square-based pyramid (C) and a “paper plane” usingthe following crosslinked polyimides, PAm-CP2-5 and PAn-CP2-1,respectively to demonstrate generality of the process. Pam-CP2-5contains 5 mol % of reacted triamine crosslinker and Pan-CP2-1, 1 mol %of reacted trianhydride crosslinker.

Additionally, as shown in FIG. 3, dynamic mechanical analysis of linearCP2 and various crosslinked CP2 films was used to compare their moduliand relative dimensional stability on the glass-transition plateau,which shows the dramatic increase in storage modulus imparted bycross-linking with the multi-functional crosslinking agents.

Repeatability of the shape recovery process was determined via cyclicDMA recovery experiments at constant heating/cooling rates. A force wasapplied at 280° C. that allowed the sample to stretch to 35% strain atwhich point the sample was equilibrated at 280° C. The sample was thencooled to 80° C., the stress released and then heated to recover theshape at 280° C. FIG. 4 shows both the three-dimensional andtwo-dimensional shape memory behaviors of the 5 mol % crosslinkedpolyimide (PAn-CP2-5), where the glass transition temperature was usedas the triggering temperature. Both shape memory fixity and recovery ofPAn-CP2-5 were also calculated based on the above tests. Shape fixity of100% implies a perfect retention of the programmed strain after theexternal stress has been released, and reflects the efficacy of thefirst two steps of the shape memory process. Shape recovery of 100%implies perfect recovery of the permanent shape after the shape memorycycle. The films were subjected to 21 testing cycles (see FIGS. 5A and5B). These films demonstrated excellent shape memory properties andrepeatability. Shape fixity is about 95.6% to about 95.7%, and shaperecovery is about 98.0% in each cycle.

While the present invention has been illustrated by the description ofone or more embodiments thereof, and while the embodiments have beendescribed in considerable detail, they are not intended to restrict orin any way limit the scope of the appended claim to such detail.Additional advantages and modification will be readily apparent to thoseskilled in the art. The invention in its broader aspects is thereforenot limited to the specific details, representative apparatus andmethods and illustrative examples shown and described. Accordingly,departures may be made from such details without departing from thescope or the spirit of the general inventive concept exemplified herein.

What is claimed is:
 1. A method for fabricating a shape memory polymerinto a three-dimensional object, comprising: a) forming a solutioncomprising an amine-terminated poly(amic acid) intermediate, wherein thesolution comprises a solvent and a reaction product obtained by reactinga stoichiometric excess of an aromatic diamine monomer and adi-anhydride-containing monomer; b) treating the solution comprising theamine-terminated poly(amic acid) intermediate with a multi-functionalanhydride crosslinking agent to thereby form a sol-gel comprising acrosslinked poly(amic acid); c) forming a film of crosslinked poly(amicacid) on a substrate to provide a laminated substrate; d) forming thelaminated substrate into a first configuration that is in athree-dimensional form; e) curing the cross-linked poly(amic acid) toprovide the shape memory polymer having a permanent shape correspondingto the first configuration, wherein the shape memory polymer comprises acrosslinked polyimide or a crosslinked poly(amide-imide); and f)removing the substrate from the laminated substrate to provide thethree-dimensional object comprising the shape memory polymer.
 2. Themethod of claim 1, further comprising: g) heating the three-dimensionalobject to a first temperature that is above a triggering temperature; h)deforming the three-dimensional object to a second configuration that isdifferent from the first configuration; and i) lowering thethree-dimensional object to a second temperature that is below thetriggering temperature while the three-dimensional object is maintainedin the second configuration.
 3. The method of claim 1, furthercomprising: j) heating the three-dimensional object in the secondconfiguration to a third temperature that is above the triggeringtemperature to thereby induce the self-rearrangement of the threedimensional object from the second configuration to the firstconfiguration.
 4. The method of claim 1, wherein forming a film ofcrosslinked poly(amic acid) on a substrate comprises: i) evaporating atleast a portion of the solvent from the sol-gel comprising thecrosslinked poly(amic acid); and ii) optionally, partially curing thecrosslinked poly(amic acid) to form the crosslinked polyimide or thecrosslinked poly(amide-imide).
 5. The method of claim 4, whereinevaporating at least a portion of the solvent further comprises heatingthe sol-gel comprising the crosslinked poly(amic acid) to a temperaturein a range of about 50° C. to about 100° C. at a pressure less thanabout atmospheric pressure.
 6. The method of claim 1, wherein thesubstrate comprises a metal that dissolves in an aqueous acid solution.7. The method of claim 6, wherein the metal comprises aluminum.
 8. Themethod of claim 1, wherein the multi-functional anhydride crosslinkingagent is defined by a general chemical formula (I):Z—(—Ar—)_(n)—W, wherein Z represents an anhydride functional group thatis directly or indirectly bonded to Ar; Ar represents an aryl group thatis directly bonded to W; and n is equal to 3 when W is P═O or N, or n isequal to 4 when W is Si or a carbon moiety.
 9. The method of claim 8,wherein Ar is a phenyleneoxy group (—OPh-) that is para- ormeta-substituted with respect to oxygen, and the agent is furtherdefined by a general formula (II):Z—(—OPh-)_(n)—W.
 10. The method of claim 8, wherein Z and Ar incombination form a phthalic anhydride moiety that is directly bonded toW.
 11. The method of claim 8, wherein Ar is a phenyleneoxy group (—OPh-)that is para- or meta-substituted with respect to oxygen, and wherein Zis a phthalic anhydride group that is connected to the phenyleneoxygroup through an ether bond.
 12. The method of claim 8, wherein W isP═O, wherein Ar is a phenyleneoxy group (—OPh-) that is para- ormeta-substituted with respect to oxygen, and wherein the agent isfurther defined by a general formula (III):Z—(—OPh-)₃—P═O.
 13. The method of claim 12, wherein Z is a phthalicanhydride group that is connected to the phenyleneoxy group through anether bond, and wherein the agent is further defined by a generalformula (V):

wherein R¹ through R⁸ are independently selected from H or C1-C4 alkyl.14. The method of claim 13, wherein the agent is selected fromtris[3-(3,4-dicarboxyphenoxy)phenyl]phosphine oxide trianhydride ortris[4-(3,4-dicarboxyphenoxy)phenyl]phosphine oxide trianhydride. 15.The method of claim 8, wherein W is N, wherein Ar is a phenyleneoxygroup (—OPh-) that is para- or meta-substituted with respect to oxygen,and wherein the agent is further defined by a general formula (VI):Z—(—OPh-)₃—N.
 16. The method of claim 15, wherein Z is a phthalicanhydride group that is connected to the phenyleneoxy group through anether bond, and wherein the agent is further defined by a generalformula (VII):

wherein R¹ through R⁸ are independently selected from H or C1-C4 alkyl.17. The method of claim 16, wherein the agent is selected fromtris[3-(3,4-dicarboxyphenoxy)phenyl]amine trianhydride ortris[4-(3,4-dicarboxyphenoxy)phenyl]amine trianhydride.
 18. The methodof claim 8, wherein W is N, wherein Z and Ar in combination form aphthalic anhydride moiety that is directly bonded to N, and wherein theagent is further defined by a general formula (VIII):

wherein R⁹ to R¹¹ are independently selected from H or C1-C4 alkyl.